In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacing and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture.” In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
The capture “treshold” is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such capture-verification algorithms, a threshold test is performed by the cardiac pacing device in order to re-determine the threshold and automatically adjust the stimulating pulse energy. This approach, called “automatic capture”, improves the cardiac stimulation device performance in at least two ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, and 2) greatly increasing the device's battery longevity by conserving the energy used to generate stimulation pulses.
Whenever a loss of capture is detected, a high-energy, safety, backup stimulation pulse is delivered to the heart within a short period of time, typically 60 to 100 ms, in order to provide backup support to the heart. The output of the primary stimulation pulse is then increased until capture is regained. Thereafter, an automatic threshold test is invoked in order to re-determine the minimum pulse energy required to capture the heart.
An exemplary automatic threshold determination procedure is performed by progressively decreasing the stimulation pulse amplitude from a functional output in small steps, for example 0.25 Volts, until capture is lost. The stimulation pulse amplitude is then increased in smaller increments, for example 0.125 Volts until capture is regained for two consecutive primary pulses. The output setting at which stable capture is regained is determined as the capture threshold. The stimulation pulse output is then adjusted to a setting equal to the threshold plus a working margin that allows small fluctuations in threshold to occur without frequent losses of capture.
The stimulation pulse amplitude will therefore be adjusted as fluctuations in threshold occur when automatic capture is enabled. Fluctuations in threshold can result from an instable electrode placement, a fracture of the conducting lead, or a discontinuity in the lead insulation. Fluctuations in threshold may also indicate a change in the patient's clinical condition.
It would be desirable to provide a diagnostic tool capable of distinguishing between safety backup pulses delivered at a high output setting from primary stimulation pulses delivered at the same high output setting. This information would be useful in assessing the performance of automatic capture verification by documenting how often safety, backup pulses are required. This information would also be valuable to a clinician in selecting programmed output settings and working margins. It would be desirable, therefore, to provide an implantable cardiac stimulation device and associated method capable of performing automatic capture verification with histogram storage of stimulation output for documenting not only the number of pulses delivered at each output setting but also the number of high-energy backup pulses.